Methods of making gas impervious sheet material



Nov, 22, 1960 R. B. SNYDER 2,961,342

METHODS OF MAKING GAS IMPERVIOUS SHEET MATERIAL Filed Aug. 15, 1957 /222 1 20 26 OVEN 32 K 3 22a L 200. 3 4 7 OVEN I81) 4 m Y OVEN 7 W 76 2 H63 F/G f4 26 50 5 3% Arm.

United METHODS OF MAKING GAS I1VIPERVIOUS SHEET MATERIAL Filed Aug. 15,1957, Ser. No. 678,289

8 Claims. (Cl. 117-76) This invention relates to methods of making gasimpervious sheet material. More particularly, it relates to such methodsof making sheet material having a high degree of impermeability towatervapor and other light gases.

It has been discovered that the thickness of the insulation required inrefrigerators and like'structures can be materially reduced by employingas the insulating material an insulating unit having a pillow-likeconstruction and comprising an outer flexible hermetically sealedenvelope containing a porous hat of glass fibers together with a gashaving a low coeflicient of heat transfer such asdichloro-difluoromethane in sufiicient quantity to provide substantiallyatmospheric pressure in the envelope.

Although it has been possible heretofore to provide flexible sheetmaterials from which to form the envelope which are substantiallycompletelyimpermeable to the dichlor'o-difluoromethane, the sheetmaterials available for this purpose have not possessed the same degreeof impermeability to the lighter gases present in the atmosphereincluding water vapor as Well as the materials which exist only in thegaseous state at normal temperatures and pressures. As a consequence,there is over a period of time appreciable leakage of these light gases,including water vapor, into the envelope 'Which ta'ken with the lack ofany leakage of the dichloro-difluoromethane from the envelopes producesan' appreciable increase in the total pressure within the envelope. Thisis objectionable not only because the increase inpressure increases thedanger that the envelope will beruptured at high temperatures but alsofor the further reasons that the increase in pressure increases thecoefficient of heat transfer of the envelope and the presence of thewater vapor within the envelope leads to condensation and, as aconsequence, to a further increase in the coefficient of heat transfer.

It is an object of this invention to provide improved methods of makingmulti-ply sheet material having a high degree of impermeability to lightatmospheric gases including water vapor as well as to heavier gases suchas dichloro-difluoromethane.

A further object is to provide improved methods of making multi-plysheet material suitable for use in the forming of heat insulatingenvelopes as above described which comprises sheet materials which incombination have a high degree of impermeability to light atmosphericgases, including water vapor, as well as to heavier gases and in whichone ply is coated directly on the other without any intervening bondinglayer.

Still another object is to provide an improved method of, making suchmulti-ply sheet materials by coating one 'are accomplished by employingas a base 'a flexible sheet of material having a uniform thickness ofthe Patent order of 1 to' l /zmils and formed, for example, ofcellophane to which has been applied a thin anch'oring coatof'vinylidene chloride copolymer such as the cellophane sheeting soldunder the trademarks:' DuPont K202 or Olin OX. Other suitable basematerials include vinylidene chloride copolymer films (such as thosesold under the trademark Saran) vinylalcohol film, vinyl chloride filmor a vinylidene chloride copolymer film. Also, the base material may beformed of the'polyesterof ethylene glycol and terephthalic acid, soldunder the tradename Mylar, a high density polyethylene, etc. Further, itis very advantageous that the base sheet be of composite constructioncomprising a sheet of kraft paper seal coated with one of the organicresin materials mentioned, or a composite laminate of two of suchresins. Of course, the resinous coating sealing the sheet ofkraft paper,or the exterior resin layer of the composite laminate, must be formed ofa resinous material that is compatible with the applied vinylidenechloride copolymer layer and to which the applied vinylidene chloridecopolymer layer may be directly adhered readily when applied directlythereto from a coating solution of the character described hereinafter.In the arrangement, wherein the base sheet is formed of kraft paper, itis recommended that the kraft paper be super-calendered and of a gaugeof about 40#, and that the resinous coatings be of the characterpreviously described;

At least one side, and preferably both sides, of the flexible base iscoated directly with a layer of vinylidene chloride copolymei' such asthe well-known copolymers of vinylidene chloride and vinyl chloride orvinylidene chloride and acrylonitrile also in a uniform thickness of theorder of l to 1 /2 mils. These relatively thick coatings are obtained bycoating with a solution of the copolymermaterial which gelssubstantially immediately upon application'so' that the coating showssubstantially no tendency to flow. Coatings having a thickness of theorder of 0.7 to 0.8 mil are readily applied in this Way in a singleapplication so that two applications are suflicient to provide thedesired final thickness, the first coat being dried completely beforethe second is applied.

The coating solution is preferably formulated to be readily flowableonly when it is heated and then maintained at elevatedtemperature'duringapplication. Such a solution may be prepared,'for example, by dissolvingthe vinylidene chloride copolymer resin in a suitable volatile solventand then diluting the resulting mixture with a less expensive volatilediluent which is usually not a solvent for the resin or polymer. Forexample, when using a vinylidene chloride and'acrylonitrile copolymersuch as that sold under the trademark or"des ignation Dow Resin" F242,25 parts of this resin is preferably dissolved .in 25 parts of a solventsuch 'as tetrahydrofuran which. has been heated to elevated temperature,and the resulting solution is then diluted with only suflicient diluent,such as methyl ethyl ketone or the solvent sold under the trademarkCellanese Solvent 601 (ordinarily about 50 parts) to provide a: flowablesolution at the temperature of operation. In general,

the coating solution preferably comprises a major pro 7 portion ofa'volatile vehicle for the resin, and the vehicle in turn comprises avolatile solvent for the resin, together with a proportion of a volatilediluent, to make the solu-' tion readily flowable Whenheated;

Suitable apparatus for making the film or sheet prod the accompanyingdrawing which specification and in which: I

Figure 1 is a diagrammatic elevational view of ;suit-' able apparatusfor making the sheet material;

Figure 2 is a view in cross section on the line 22 of Figure 1;

Figure 3 is a view in cross section on the line 33 of Figure 1;

Figure 4 is a view in cross section on the line 4--4 of Figure l; and

Figure 5 is a view in cross section on the line 55 in Figure 1.

Referring first to Figure 1, it will be seen that there is provided asupply roll of the base sheet material, such as cellophane coated onboth faces with a thin layer of vinylidene chloride copolymer as above,suitably mounted to permit a continuous web 12 to be unwound from theroll. From supply roll 10 the web 12 is drawn over a rotatable roll 14forming part of a knife coater designated generally 16 and including ahopper 18 mounted above the roll 14 and provided with a knife edge 20below the outlet 22 of the hopper. A body 24 of coating solution such asis described above is placed in the hopper 18 and the knife edge 20 isspaced above the roll 14 to meter the coating applied to the web 12through the outlet 22 to the proper thickness to form a layer 26 havinga thickness when dried of approximately 0.7 mil. The solution 24 ispreferably heated to a. temperature of about 130 to 140 F. before it isfilled into hopper 18 and is also preferably maintained at a temperaturein the same range while it is in the hopper by suitable heating means(not shown).

The web 12, after passing the knife edge 20 and receiving the layer 26of vinylidene chloride copolymer resin solution, is drawn through anoven 28 provided with a source (not shown) of infrared heat. The oven 28is constructed and arranged in a known manner completely to dry thelayer 26.

From the oven 28 the web 12 is drawn around a pair of rotatable guiderolls 30 and 32 rotating in the directions indicated by the respectivearrows shown thereon, after which it passes through a second coatingdevice 16a identical with that described above. The like parts of thissecond coating device are numbered in the drawing with the same numbersapplied to the parts of the device first described but followed by theletter a and function in the same way.

As the web 12 passes the knife edge 20a, a second layer 34 of thevinylidene chloride copolymer resin solution is applied thereto over thelayer 26 to provide the desired final coating thickness on that face ofthe web 12 and is dried completely as the web 12 is then drawn,

through the oven 28a which is identical with the oven 28.

Two more rotatable guide rolls 36 and 38 direct the web 12 from oven 28ato a third coating device 16b and then through a third oven 28!). Stillanother pair of rotatable guide rolls 40 and 42 finally guide the web 12past a fourth coating device 16c and through a final oven 280. Each ofthe coaters 16b and 160 is also identical with the coater 16 andcorresponding parts bear the same number with a further identifyingletter. Similarly, ovens 28b and 280 are identical with oven 28 abovedescribed.

As may be seen the coater 16b applies a third layer 44 of the vinylidenechloride copolymer resin solution to the face of the web 12 opposite tothat on which layers 26 and 34 Were applied and coater 16c deposits afourth layer 46 on top of the layer 44.

After leaving the oven 280, the web 12 with the dried layers ofvinylidene chloride copolymer resin thereon is wound about a rotatableroll 48 which is driven in the direction indicated by the arrow shownthereon and serves to pull the web through the four coaters and the fourovens. When a suflicient thickness of the coated web is wound around theroll 48, it is removed and packaged or stored as desired.

As indicated in Figure 2 of the drawing, the web of cellophane has athickness of 0.0013 inch, or 1.3 mils, and has applied to it in thefirst coater through the appropriate setting of the knife edge 20 alayer of the vinylidene chloride copolymer resin solution which has athickness when dried of approximately 0.0007 inch, or 0.7 mil. In thesecond coater, the knife edge 20a is set to provide a coating of similarthickness so that the sheet as shown in Figure 3 prior to passingthrough the coater 16b consists of the base layer 12 and a compositelayer 50 of the vinylidene chloride copolymer resin formed by the mergeron drying of the two layers 26 and 34 having a dried thickness ofapproximately 0.0014 inch, or 1.4 mils, and adhering to the base 12.Similarly, layers 44 and 46 are applied in the same thickness and mergeon drying to form a composite layer 52 of the copolymer as shown inFigures 4 and 5.

It is believed that the operation of the apparatus illustrateddiagrammatically in Figure 1 will be readily apparent. A supply ofcoating solution, such as is described above, is placed in thecontainers 18, 18a, 18b and 18c and a roll of cellophane as described isprovided as the supply roll 10. A web 12 unrolled from this roll isthreaded through the coaters and the ovens and secured to the winduproll 48. The means for driving the roll 48 is then put into operationand as the web 12 is pulled through the coaters and ovens the desiredlayers of the vinylidene chloride copolymer resin are applied to thebase and dried in the manner above described.

The invention is not limited, however, to the particular coatingmechanism which has been described. Any desired type of roll coater orknife coater or other coating device which is capable of applying acoating of the coating solution in the desired thickness to a. base suchas the base 12 may be employed for the purpose of this invention. Anydesired means for drying the applied layer of coating solution, i.e.,evaporating volatile solvents, may be employed for the purpose of thisinvention although it is preferred to employ infrared heating asdescribed. It is also preferable to gradually increase the heat as theweb moves through the oven preferably reaching a temperature of theorder of 300 F. with the combination of materials used as anillustration since this provides more efficient drying of the coating.Four ovens are shown in the drawings for convenience of illustration butit should be understood that it is possible by the provision of suitableguide rolls to direct the web through the same oven more than once forthe purpose of drying a plurality of coatings which are applied to givethe desired final thickness.

It will also be obvious that instead of employing the windup roll 48 todraw the web 12 through the coaters and ovens any other desired meansmay be employed for this same purpose. For example, a pair ofcooperating rolls, at least one of which is driven, may be positioned soas to grip the web after the final coating has dried and to provide thedesired pull thereon. In fact, such an arrangement has the advantagethat the problem of variation in lineal speed of the web resulting fromvariations in the diameter of the windup roll 48 is there by avoided.

Although heating means for the containers 18, 18a, 18b and 18c are notshown in the drawing, it is preferable and advantageous to heat thecoating solution to keep it warm as it is applied. This preventsgelling, reduces the viscosity of the coating solution, and alsoeliminates condensation of water vapor during the coating operation.High temperature also increases the solvating tendency of the diluentpreferably employed in the coating solu tion and makes the coatingsolution more stable. Heating the coating solution also makes itpossibleto employ a more concentrated solution which gels rapidly afterit is applied so the tendency of the applied coat to flow is minimized.

The preferred coating solution which, as indicated above, includes botha solvent for the resin and a diluent, preferably has a concentration ofresin as high as possible to permit coating of 50% by weight. It iswithin the scope of the invention to employ both higher and lowerconcentrations of resin. For example, when concentrations as high as 30%to 50% are employed, such heavy solutions should be maintained underclosely controlled elevated temperature in order to provide a desirableviscosity without excessive evaporation.

As indicated above, the invention contemplates the application to one ofthe bases described of any desired vinylidene chloride copolymer resin.Furthermore, it is within the scope of the invention to employ anydesired solvents and diluents and to vary their relative proportion solong as the resulting coating solution has the proper characteristics topermit it to be coated on to a base as described.

It is an outstanding advantage of the present invention that it providesfor the first time a composite film or sheet consisting of a flexiblebase (formed of cellophane or other suitable material) having firmlysecured thereto on one or both faces a relatively thick layer of avinylidene chloride copolymer resin material thereby providing acomposite sheet material which has a high degree of impermeability, bothto the light atmospheric gases such as oxygen and nitrogen, and also towater vapor. The film of vinylidene chloride-acrylonitrile copolymer ismost advantageous, as it is much more impervious (about ten times) thana corresponding film of vinyl chloride-vinylidene chloride copolymer.The invention has the further advantage that it provides for the firsttime a method by which such a composite sheet may be made.

Such a sheet has perhaps its highest utility when it is employed to formthe outer flexible hermetically sealed bag or envelope for a heatinsulating unit such as is described above. Such envelopes and theirformation, as well as their use for that purpose in the void defined inassociated hollow structure, are described in a copending application ofThomas Steve Simms, et al., Serial No. 504,248, filed April 27, 1955,which points out that these heat insulating units when used inrefrigerator constructions, for example, may be subject to a normaloperating temperature range extending from about 30 F. to about +l50 F.

As indicated in that application, an envelope of the type described maybe formed by taking a portion of a flexible composite sheet, such as isdescribed in the present application, of the required area andconfiguration and folding it about a deformable porous mass of solidpoor heat conducting material, such as a porous bat of glass fibres,with the vinylidene chloride copolymer layer forming the inner wall,after which the opposed edges are sealed together around the three sideswhich remain open, with a suction tube of a suitable thermoplasticmaterial extending through between the sealed together edges at anydesired point. As set forth in the above copending application, thecontainer or bag thus formed, in which the solid poor heat conductingmaterial is confined so that it generally fills out the volume thereof,is evacuated through the aforesaid tube and the air so withdrawn is thenreplaced with a gas such as dichlorodifluoromethane having a thermalconductivity or coefiicient of heat transfer lower than that of air,after which the thermoplastic tube is heat sealed to complete theclosure of the envelope. The charge of gas is such as to complete thefill of the bag and thoroughly permeate through the porous mass. It hasbeen found that such an envelope is not only eflfective to retain thecharge of low thermal conductivity gas therein, thus providing a veryeffective heat insulator, but at the same time the imperviousness of theenvelope, both as to water vapor and other light atmospheric gases,minimizes the leakage of such gases into the interior of the envelope sothat its initial heat insulating quality is maintained to a high degreeover long periods of time and under adverse conditions.

It is apparent that many widely different embodiments of this inventionmay be made without departing from the spirit and scope thereof and is,therefore, not intended to be limited except as indicated in theappended claims.

What is claimed is:

l. The method of making a composite sheet comprising applying to onesurface of a flexible base sheet a uniform first layer of a solution ofvinylidene chloride copolymer resin in a volatile solvent, whichsolution gels substantially immediately upon application, said solutionbeing applied in said first layer in an amount suflicient to provide afirst coating having a uniform thickness of the order of 0.5 to 0.75 milwhen dry, drying said applied first layer, applying to said dried firstcoating a uniform second layer of said solution to provide a secondcoating having a similar thickness when dry, and drying said appliedsecond layer.

2. The method set forth in claim 1, wherein said base sheet is formed ofan organic resin.

3. The method set forth in claim 1, wherein said base sheet is formed ofcellophane having an initial relatively thin anchoring coating ofvinylidene chloride copolymer resin on the surface thereof to which saidfirst layer is applied.

4. The method set forth in claim 1, wherein said base sheet is formed ofkraft paper seal-coated with an organic resin over which said firstlayer is applied.

5. The method set forth in claim 1, wherein said solution is at anelevated temperature when said layers are applied therefrom.

6. The method set forth in claim 1, wherein said solution comprises amajor proportion of a volatile organic vehicle for said resin, and saidvehicle comprises a volatile organic solvent for said resin and avolatile organic diluent for said solvent.

7. The method set forth in claim 1, wherein said layers are dried byexposure to infrared heat.

8. The method set forth in claim 1, wherein said solution comprises anorganic solvent for said resin and an organic diluent for said solvent,and said solution is at an elevated temperature when said layers areapplied therefrom.

References Cited in the file of this patent UNITED STATES PATENTS1,969,621 Munters Aug. 7, 1934 2,067,015 Munters Jan. 5, 1937 2,386,700Manchester Oct. 9, 1945 2,618,575 Oswin Nov. 18, 1952 2,704,732 Copemanet al. Mar. 22, 1955 2,779,066 Gaugler et a1 Jan. 29, 1957 2,819,984Ackerman Ian. 14, 1958 2,824,025 McIntyre Feb. 18, 1958

1. THE METHOD OF MAKING COMPOSITE SHEET COMPRISING APPLYING TO ONE SURFACE OF A FLEXIBLE BASE SHEET A UNIFORM FIRST LAYER OF A SOLUTION OF VINYLIDENE CHLORIDE COPOLYMER RESIN IN A VOLATILE SOLVENT, WHICH SOLUTION GELS SUBSTANTALLY IMMEDIATELY UPON APPLICATION, SAID SOLUTION BEING APPLIED IN SAID FIRST LAYER IN AN AMOUNT SUFFICIENT TO PROVIDE A FIRST COATING HAVING A UNIFORM THICKNESS OF THE ORDER OF 0.5 TO 0.75 MIL WHEN DRY, DRYING SAID APPLIED FIRST LAYER, APPLYING TO SAID DRIED FIRST COATING A UNIFORM SECOND LAYER OF SAID SOLUTION TO PROVIDE A SECOND COATING APPLIED SECOND LAYER. 